1. Field of the Invention
The invention is concerned with fibers for use as transmission lines in communications systems operating in the visible or near visible spectra. Such fibers are generally clad for guiding purposes so that refractive index decreases in value from the core center to the periphery either as a step function or as a continuous gradient.
2. Description of the Prior art
"Optical" communications systems, that is systems operating in the visible or near visible spectra, are now at an advanced stage of development. In accordance with the view held by many, commercial use may be expected within a period of about five years.
A system most likely to find initial, and probably long term, use utilizes clad glass fibers as the transmission medium. These fibers, generally having an overall cross-sectional diameter of about 100 .mu.m, are generally composed of at least two sections: core and cladding. The cladding layer is necessarily of lowered refractive index relative to the core with typical index variation from core to clad being in the range from about 0.01 to 0.05. Structures under study may be single mode or multimode. The former is characterized by a sufficiently small core section to efficiently accommodate only the first order mode. Such structures may have a core about 1 or 2 .mu.m. Multimode lines typically have core sections from 50 .mu.m to 85 or 90 .mu.m in diameter.
Multimode structures appear to be of somewhat greater interest at this time. The greater core section facilitates splicing and permits more efficient energy coupling to source and repeater. Introduction of many modes into or, alternatively, generation of many modes within the line does give rise to a dispersion limitation which takes the form of a smearing due to the differing velocities of different order modes. Mode dispersion effects have been minimized by a continuous focusing structure. This structure takes the form of a fiber, the index of which is graded generally exponentially from a high value at the core center. The fundamental mode which traverses the length of material is generally confined to the highest index (lowest velocity) region, while higher order modes as path length increases spend longer and longer periods in relatively low index (high velocity) regions.
A number of procedures have been utilized for fabricating clad glass fibers. Most have yielded to procedures which in some way involve vapor source material. So, typically, chlorides, hydrides, or other compounds of silica, as well as desired dopants, tailoring the index, are reacted with oxygen to produce deposits which directly or ultimately serve as glass source material from which the fiber is drawn. Dopant materials include compounds of, for example, boron for lowering index and germanium, titanium, aluminum, and phosphorus for increasing index. Where the ultimate product is to be a graded multimode line, index gradation may be accomplished, for example, by altering the amount or type of dopant during deposition.
One procedure utilizing vapor source material is chemical vapor deposition (CVD). In this procedure, compounds are passed over a heated surface--e.g., about a rod or within a tube. Temperatures and rates are adjusted so that reaction is solely heterogeneous, i.e., occurs at the heated surface so that the initial material is a continuous glass layer.
An alternative procedure results in the introduction of such precursor materials into a flame produced by ignition of a gaseous mixture of, for example, methane and oxygen. Reaction is, in this instance, homogeneous resulting in formation of glassy particles within the flame. Combustion product and glassy particles then form a moving gas stream which is made incident again on a heated surface, such as a rod or tube. Adherent particles sometimes called "soot" are in subsequent processing flushed, and are sintered and fused to result in a glassy layer.
The CVD process has advantages including high purity but suffers from prolonged required deposition periods. Typically, a suitable preform adequate for fabrication of a kilometer of fiber may require periods of a day or longer.
The soot process has the advantage of high speed; preforms adequate for fabrication of a kilometer of fiber may be prepared in a few hours or less. Disadvantages, however, include at least initial introduction of contaminants, such as solid carbonaceous residue. Since formation takes place within the combustion environment, hydration is inevitable; and this gives rise to the well-known water absorption peaks with their related sub-harmonics so consequential in various portions of the infrared spectrum.
Both procedures are now an established part of the art. See, for example, U.S. Pat. Nos. 3,711,262, 3,737,292, and 3,737,293. Modifications in the processes have, to some extent, increased the speed of the CVD process and reduced the effects of contamination by hydration in the soot process. Fibers a kilometer or more in length with losses as low as 2 or 3 dB/kilometer in selected regions of the infrared are now regularly produced in pilot operations.